Add swift_store_auth_version to swift backend.

Updates the glance swift backend to support configuring the
swift_store_auth_version setting. The default value for this is '2'
(which matches the glance config file defaults).

As part of this change I am also updating the default value
for swift_store_auth_address to use 127.0.0.1:5000/v2.0/ (keystone).
This makes sense because the previous value would have required
us to set swift_store_auth_version by default to 1.

Change-Id: I865712914dd0506b2722ab8da3e1c56d7d9f433a
Merge "Add swift_store_auth_version to swift backend."